lsomers as a probe of triple shape co-existence in neutron-deficient Pb nuclei

Full text: Nuclei have a surprising ability to exhibit distinct co-existing shapes, controlled by non-uniformities in the single-particle level densities near the Fermi surface and through the population of specific orbitals by valence protons and neutrons. Although lead nuclei are generally spherical because of the closed proton shell at Z=82, the coexistence of three shapes at low energy, spherical, oblate deformed and prolate deformed are predicted by macroscopic/microscopic theories, to occur near neutron number N=106. Distinguishing between such shapes through level structures is generally ambiguous at low angular momentum but the presence of metastable states with different spins and parities and decay properties may provide a means of characterisation. We have offered such evidence recently for the case of 188Pb and have now carried out an extensive measurement using time correlated gamma-gamma coincidence techniques with the state-of-the art array, GAMMASPHERE at LBNL. Collective structures above the isomers have been observed and the fragmented decay of the proposed prolate isomer has provided a means of identifying non-yrast states which were previously inaccessible. The implication of the new results including features such as electric monopole transitions between coexisting bands will be discussed